Method for Direct Deposition of a Germanium Layer

ABSTRACT

The present disclosure is related to a method for the deposition of a continuous layer of germanium on a substrate by chemical vapor deposition. According to the disclosure, a mixture of a non-reactive carrier gas and a higher order germanium precursor gas, i.e. of higher order than germane (GeH 4 ), is applied. In an example embodiment, the deposition is done under application of a deposition temperature between 275° C. and 500° C., with the partial pressure of the precursor gas within the mixture being at least 20 mTorr for temperatures between 275° C. and 285° C., and at least 10 mTorr for temperatures between 285° and 500° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application Ser. No.11150559.0 filed Jan. 11, 2011, the contents of which are herebyincorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure is related to semiconductor processing, inparticular to the deposition of Germanium layers by Chemical VaporDeposition (CVD).

STATE OF THE ART

According to the present state of the art, the CVD-deposition ofpolycrystalline or amorphous Ge-layers on a SiO₂ layer using germane(GeH₄) as the Ge-precursor is not possible without the deposition of asilicon seed layer on the SiO₂ layer. Without the seed layer, noGe-growth of a continuous, i e uniform and closed Ge layer is possibledue to the formation of germanium suboxides (by reaction between the Geand oxide) which are volatile and therefore desorb at the growthtemperature. On a substrate comprising areas of Si and areas of SiO₂,the Ge-growth is thus said to be selective, i.e. a continuous Ge isgrown on the Si-areas but not on the SiO₂ areas.

The presence of a Si-seed layer is undesirable in some applications, buta method for growing a pure and continuous Ge layer directly on Si0₂ isnot available at present.

Instead of GeH₄, higher order germanium precursors, in particulardigermane (Ge₂H₆) and trigermane (Ge₃H₈) have been applied incombination with Si-precursors in the CVD-production of SiGe layers.Document US-A-2003111013 for example describes a process and apparatuswhich allows SiGe deposition on SiO₂ without a seed layer, and utilizingone of GeH₄, Ge₂H₆ Ge₃H₈ or tetrachlorogermane (GeH_(1-x)Cl_(x), x=1-4)as the Ge-precursor and one of silane (SiH₄), disilane (Si₂H₆) trisilane(Si₃H₈) or tetrachlorosilane (SiHCl₄) as the Si precursor. No indicationis given as to the process parameters for producing a pure Ge layerwithout applying a seed layer.

It is concluded that the prior art lacks a method for depositing pure Gelayers on SiO₂. The present disclosure aims to provide such a method.

SUMMARY OF THE DISCLOSURE

The disclosure is related to a method and to devices as disclosed in theappended claims. As such, the disclosure is related to a method fordepositing a continuous germanium layer on a substrate surface,comprising the steps of :

-   -   Providing a substrate,    -   Introducing said substrate into a reaction chamber suitable for        applying a layer onto said surface by chemical vapor deposition        (CVD),    -   Introducing in said chamber a gas mixture comprising a germanium        precursor gas and a non-reactive carrier gas, said germanium        precursor gas being a higher order germane precursor compared to        GeH₄,    -   Depositing by CVD a continuous germanium layer overlying and in        contact with said substrate surface.

According to example embodiments, said germanium precursor is Ge₂H₆ orGe₃H₈. Said gas mixture may be at atmospheric pressure.

According to an example embodiment, said deposition step is performed ata deposition temperature between about 275° C. and about 500° C., andthe partial pressure of the germanium precursor gas in said gas mixtureis at least 20mTorr when the deposition temperature is between about275° C. and about 285° C., and at least 10 mTorr when the depositiontemperature is higher than about 285° C. and up to about 500° C. The CVDdeposition could take place by performing the following steps:

-   -   heating the interior of said reaction chamber to a deposition        temperature, and possibly while already supplying said carrier        gas to the chamber,    -   when the deposition temperature is reached, supplying the        precursor gas and the carrier gas to the chamber, resulting in        the deposition of the Ge layer on the substrate.

According to embodiments of the disclosure, said substrate surfacecomprises at least a surface area consisting of a material of the groupconsisting of:

-   -   high-K dielectric materials,    -   Silicon oxide,    -   Si-nitrides or carbides,    -   metals or metal nitrides or metal carbides.

According to specific embodiments said substrate surface comprises atleast a surface area consisting of titanium nitride and/or least asurface area consisting of SiO₂.

Said gas mixture may further comprise a gas containing a doping element.Said gas containing a doping element may be B₂H₆ or AsH₃ or PH₃. Saidcarrier gas may be H₂ or N₂.

The disclosure is equally related to a substrate comprising a TiN layerat least on a part of the substrate surface and a continuous Ge layeroverlying and in contact with said TiN layer.

The disclosure is equally related to a semiconductor device comprising asubstrate according to the previous paragraph. The disclosure is equallyrelated to a semiconductor device comprising a substrate produced by themethod of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a reactor suitable for the method ofthe disclosure.

FIG. 2 shows the Ge growth rate as a function of the partial pressure ofGe₂H₆ at several deposition temperatures.

FIGS. 3 a and 3 b are SEM pictures illustrating the fact that nocontinuous Ge layer can be deposited on TiN starting from GeH₄ as theprecursor, with or without a Si seed layer.

FIG. 4 demonstrates the formation of a continuous Ge layer directly onTiN, according to the method of the disclosure.

DETAILED DESCRIPTION

According to the method of the disclosure, a higher order Ge-precursor,such as digermane or trigermane (Ge₂H₆ or Ge₃H₈) is used as thegermanium precursor gas in a gas mixture, which may include a gasmixture at atmospheric pressure, comprising or consisting of the Geprecursor gas, possibly a gas comprising dopant elements, and anon-reactive carrier gas, for example N₂ or H₂. The dopant element couldbe boron added to the mixture as B₂H₆ in gaseous form. Also n-typedoping elements could be added, for example by adding AsH₃ or PH₃ to themixture. From this mixture, a (possibly doped) continuous Ge layer isdeposited by CVD on a substrate surface, e.g. on SiO₂, without requiringa Si seed layer. The Ge layer can be amorphous, polycrystalline ormonocrystalline, mainly depending on the type of substrate onto whichthe layer is deposited. By a continuous layer is meant a layer that doesnot show island formation. The deposition method as such is conductedaccording to known CVD procedures, and may be executed in known CVDinstallations. The CVD technique applied in the method of the disclosuredoes not make use of plasma-assisted processes such as plasma enhancedCVD (PECVD).

The method of the disclosure is applicable on any type of substratesurface. According to the example embodiments, a Ge layer is depositedon a material on which it is impossible to directly deposit a continuousGe layer by CVD from a gaseous phase comprising GeH₄ as theGe-precursor. The main example of such a material is silicon oxide, inparticular SiO₂. Other surfaces with which the disclosure could beapplicable are layers of the following materials:

high-K dielectric materials, such as hafnium oxides, zirconium oxides,lanthanides oxides (lanthanum oxide, dysprosium oxide, gadolinium oxide,ytterbium oxide) and any combination thereof. These materials could showthe same type of potential problems as silicon oxide in terms of thedirect deposition of Ge with GeH₄ as the precursor, i.e. the formationof volatile Ge suboxides which desorb at the growth temperature,

Si-nitrides or carbides (SiN, SiC),

metals or metal nitrides/carbides (e.g. Hf, Zr, Al, Ti, TiN, Ta, Zr, Ru,TaN, TaC).

Metal nitrides or carbides could show problems when depositing Ge withGeH₄ as the precursor, even when a Si seed layer is deposited first. Theinventors have established such problems in the case of a TiN surface(see further). Without wishing to be bound by theory, such problemscould be caused by the presence of metal contamination at the Si/Geinterface.

Hereafter test results are described which were obtained on a siliconwafer of 200 mm in diameter having a surface comprising areas of Si andareas of SiO₂. The precursor used was Ge₂H₆ with H₂ as the carrier gas.These results are however applicable to other types of substrates, andother higher order Ge-precursors, such as Ge₃H₈, as well as othercarrier gases (e.g. N₂).

The CVD deposition was done in a CVD reactor as schematicallyillustrated in FIG. 1 and not meant to limit the scope of thedisclosure. The carrier gas and precursor gas are provided towards theinlet 1 of a reaction chamber 2, in which the substrate 3 is mounted. Aheating system is provided to heat up the reactor and the substrate to adesired temperature. First the interior of the reaction chamber isheated to the deposition temperature, and in some embodiments, while aflow of the carrier gas is already supplied to the chamber. Suitablevalve means (not shown) are provided to control the flow of gasestowards the chamber. When the deposition temperature is reached, theprecursor gas is added, and the mixture of the precursor and the carriergas is supplied to the chamber, resulting in the deposition of the Gelayer on the substrate 3. Gases exit from the chamber through the outlet4. In the tests of which the results are described hereafter, a flowrate of 20 l/min of H₂ carrier gas was supplied. The flow rates of thecarrier gas and the precursor gas in their respective supply sectionsdetermine the partial pressure of the precursor gas in the mixture bythe formula:

$p_{p} = {\frac{{FR}_{p}}{\sum F}*p_{m}}$

with: p_(p) the partial pressure of the precursor gas, FR_(p) the flowrate of the precursor gas (taking into account precursor dilution), ΣFis the sum of all the flows in the chamber (all precursor gases+carriergas), p_(m) the total pressure in the reactor. Said total pressure maybe atmospheric pressure or lower than atmospheric pressure. Theapplication of the method at atmospheric pressure offers the advantagethat higher partial pressures can be obtained for the same flow rates.

In FIG. 2, the growth rate of Ge on Si is shown as a function of theGe₂H₆ partial pressure for different deposition temperatures. The line10 represents the separation between test results showing selectivegrowth, i.e. no nucleation on SiO₂ to the left of line 10, and testresults showing non-selective growth, i.e. nucleation on SiO₂, to theright of line 10.

At temperatures lower than 275° C., Ge growth becomes so low as to beunworkable, on Si as well as on SiO₂. Above about 500° C., the growthwas selective in every case, i.e. nucleation on Si but not on SiO₂.Between about 275° C. and 500° C., non-selective Ge growth could beobtained depending on the partial pressure of Ge₂H₆. At the lower end ofthe temperature range of 275° C.-500° C., a higher partial pressure isrequired (at least 20 mTorr), while starting from about 285° C., thepartial pressure must be at least 10 mTorr. The maximum of the partialpressure is determined by practical circumstances (e.g. sufficient flowof carrier gas) and can differ as a function of the actual installationand materials used. These results prove that there is a window in termsof the deposition temperature, between about 275° C. and about 500° C.,within which a continuous layer of pure Ge can be deposited directly byCVD on SiO₂, provided that the partial pressure of the high order Geprecursor is sufficiently high. The disclosure is equally related to asemiconductor device comprising a substrate produced by the method ofthe disclosure.

The method of the disclosure is especially applicable to the depositionof a continuous Ge layer on titanium nitride (TiN). It was found by theinventors that on TiN it was not possible to deposit Ge by using germane(GeH₄) as the precursor, not even with the application of a Si seedlayer. Tests were done wherein Ge was deposited by CVD on a Si substrateprovided with a TiN layer of about 100 nm thickness. In a first test, Gewas deposited directly on the TiN layer. In two further tests, Ge wasdeposited after deposition of a Si seed layer of about 40 nm thicknessand in another test after deposition of a Si seed layer of about 85 nmin thickness. In each case, Ge growth occurred by formation of pillarsor islands, not in a continuous layer. This is illustrated in FIGS. 3 aand 3 b. FIG. 3 a shows the Si substrate 20, provided with TiN layer 21and Ge layer 22 deposited from GeH₄ directly on the TiN. It is seen thatthe Ge layer is very irregular and not continuous. FIG. 3 b shows theeffect of a Si seed layer 23 of 86 nm. The Ge formation is still in theform pillars and islands 24. Without wishing to be bound by theory, itis believed that such problems are caused by metal contamination at theSi/Ge interface. Similar problems are expected for deposition on othertypes of metal, metal nitride or metal carbide surfaces.

In a comparative test, a Si substrate provided with a TiN layer of 98.2nm thick was subjected to the method of the disclosure, using Ge₂H₆ at300° C., H₂ as carrier gas, the total pressure being atmosphericpressure and the partial pressure of Ge₂H₆ being 221 mTorr, in theinstallation as schematically shown in FIG. 1. The result as illustratedby FIG. 4, was a continuous layer 25 of polycrystalline Ge, overlyingand in contact with the TiN layer 21. The disclosure is thus alsorelated to a semiconductor substrate comprising a TiN layer at least ona portion of the substrate surface, with a Ge layer overlying and incontact with said TiN layer. The disclosure is equally related to asemiconductor device comprising such a substrate.

1. A method, comprising: providing a substrate, wherein said substratecomprises a substrate surface; introducing said substrate into areaction chamber, wherein said reaction chamber is configured to apply alayer onto said substrate surface by chemical vapor deposition (CVD);introducing in said reaction chamber a gas mixture comprising agermanium precursor gas and a non-reactive carrier gas, wherein saidgermanium precursor gas comprises a higher order germane precursorcompared to GeH₄; and depositing by CVD a continuous germanium layer,wherein said continuous germanium layer overlies and is in contact withsaid substrate surface.
 2. The method of claim 1, wherein said germaniumprecursor gas is Ge₂H₆.
 3. The method of claim 1, wherein said germaniumprecursor gas is Ge₃H₈.
 4. The method of claim 1, wherein said gasmixture is at atmospheric pressure.
 5. The method of claim 1, whereinsaid depositing by CVD is performed at a deposition temperature between275° C. and 500° C., and wherein a partial pressure of said germaniumprecursor gas in said gas mixture is at least 20 mTorr when saiddeposition temperature is between 275° C. and 285° C., and at least 10mTorr when said deposition temperature is between 285° C. and 500° C. 6.The method of claim 1, wherein said substrate surface comprises a high-Kdielectric material.
 7. The method of claim 1, wherein said substratesurface comprises silicon oxide.
 8. The method of claim 1, wherein saidsubstrate surface comprises silicon nitride.
 9. The method of claim 1,wherein said substrate surface comprises silicon carbide.
 10. The methodof claim 1, wherein said substrate surface comprises a metal.
 11. Themethod of claim 1, wherein said substrate surface comprises a metalnitride.
 12. The method of claim 1, wherein said substrate surfacecomprises a metal carbide.
 13. The method of claim 1, wherein saidsubstrate surface comprises titanium nitride.
 14. The method of claim 1,wherein said substrate surface comprises silicon dioxide.
 15. The methodof claim 1, wherein said gas mixture further comprises a dopant gas,wherein said dopant gas comprises a doping element.
 16. The method ofclaim 15, wherein said dopant gas is a material selected from the groupconsisting of B₂H₆, AsH₃ and PH₃.
 17. The method of claim 1, whereinsaid non-reactive carrier gas is a material selected from the groupconsisting of H₂ and N₂.
 18. A semiconductor device, comprising asubstrate, wherein said substrate comprises a substrate surface, whereinsaid substrate surface comprises a titanium nitride layer, wherein acontinuous germanium layer overlies and is in contact with said titaniumnitride layer.
 19. The semiconductor device of claim 18, wherein saidcontinuous germanium layer is applied onto said substrate surface by achemical vapor deposition process.
 20. The semiconductor device of claim19, wherein said chemical vapor deposition process is performed at adeposition temperature between 275° C. and 500° C., and wherein apartial pressure of a germanium precursor gas in a gas mixture is atleast 20 mTorr when said deposition temperature is between 275° C. and285° C., and at least 10 mTorr when said deposition temperature isbetween 285° C. and 500° C.